1. Field of the Invention
The invention relates to a blade made from a composite material for a helicopter rotor whose structure is particularly well adapted not only to bending but also to twisting forces and which is further-more redundant; this quality allows the operating integrity to be maintained after structural damage. This type of structure is also well adapted to blades whose root is "forked".
2. Description of the Prior Art
It is in fact known that for forming the root of the blades and fastening them to the articulations of the hub which allow angular movements of said blades in so far as the angle of attack, flapping and drag are concerned, particularly when these articulations are formed by an angularly deformable resilient body, known under the name of "laminated spherical thrust block" and such as described in French Pat. No. 2 427 251 filed on the June 2, 1978 in the name of the assignee, it is advantageous for the blade foot to comprise two branches forming a fork whose end is bolted directed to the internal frame of the laminated spherical thrust block. This type of construction provides on the one hand a simplification of the construction of the rotor and lightening thereof by reducing the number of constituent parts and, on the other hand, allows the blade/hub connection to be placed closer to the center of oscillation of the blade, which results in an appreciable reduction of the dynamic forces applied to the fastening. This gives to this member a better fatigue strength and an unlimited lifespan.
Furthermore, compartment blades are known comprising either a single compartment in the vicinity of the leading edge or multiple compartments in the direction of the chord of the blade. The main advantage of such blades resides in their very high torsional rigidity for eliminating the vibrations due to coupling between the bending and twisting deformations of the blade. However, and more especially in the case of a blade having a fastening fork, construction thereof becomes complex while not giving entire satisfaction in so far as the strength of the connection zone of the current part of the structure to the forked root is concerned.
In fact, a structure having a single compartment which must be divided into two compartments to form the two branches of the bork presents a discontinuity in the transition zone which is very unfavorable for the passage of torsional forces in particular. For structures with multiple compartments in the direction of the chord, it is impossible to extend them into forked branch compartments without a sudden structural discontinuity of at least one compartment, resulting also in a weaked transition zone. Of course, such weakening is offset by the positioning of reinforcements, ribs, hoops and other means to try to reconstitute a continuous structure, but that results in great complication of manufacture. It follows that blades with fork and compartment are practically impossible to manufacture in a simple and so economical way.